It has been previously disclosed (see P. H. Plesch, Chem. and Ind., page 954, July 26, 1958) that graft copolymers can be prepared by the "grafting from method" using AlCl.sub.3, TiCl.sub.4, etc. catalysts to abstract chlorides from a polyvinyl chloride backbone and subsequently to introduce a monomer such as styrene which polymerizes at the carbonium ion site created on the backbone. There are many difficulties and disadvantages to this prior art technique. Some of these are: (a) only minor quantities and yields of the graft polymerization can be obtained even under the best circumstances since the kinetics of the reaction strongly favor the production of homopolymer rather than graft polymer; (b) the backbone polymer is seriously degraded; (c) the catalyst solubility is incompatible with the backbone polymer solubility, and (d) the physical properties of the resulting materials are unsatisfactory.